Four high school boys were late to their morning classes one day. They entered the classroom and solemnly told their teacher they were detained due to a flat tire. The sympathetic teacher smiled and told them it was too bad they were late because they had missed a test that morning. But she was willing to let them make it up. She gave them each a piece of paper and a pencil and sent them to four corners of the room. Then she told them they would pass if they could answer just one question:
Which tire was flat???